Alternating field-impressed AMS in rocks

We studied the evolution of anisotropy of magnetic susceptibility (AMS) during stepwise alternating fields (AF) demagnetization on various kinds of rock samples (loess and palaeosols, diorite, granite, gneiss) with very different degree of magnetic anisotropy. The variation of the magnetic fabric appears to be related to both the magnetic fabric before AF demagnetization and to the direction of AF application. The anisotropy change is mainly controlled by the initial magnetic fabric. The more anisotropic is the initial magnetic fabric, the less is the effect of the direction of field application. This can be clearly shown by determination of the difference of the susceptibility ellipsoids after and before AF application. Even for rocks with weak magnetic anisotropy, the effect of the initial AMS is significant. The difference ellipsoids allow in particular cases to point out that the initial magnetic fabric is composite.